Metal Nanoparticles Formed Around A Nucleus and Scalable Processes for Producing Same

ABSTRACT

Metal nanoparticles and compositions derived therefrom can be used in a number of different applications. Methods for making metal nanoparticles can include providing a first metal salt in a solvent; converting the first metal salt into an insoluble compound that constitutes a plurality of nanoparticle seeds; and after forming the plurality of nanoparticle seeds, reacting a reducing agent with at least a portion of a second metal salt in the presence of at least one surfactant and the plurality of nanoparticle seeds to form a plurality of metal nanoparticles. Each metal nanoparticle can include a metal shell formed around a nucleus derived from a nanoparticle seed, and the metal shell can include a metal from the second metal salt. The methods can be readily scaled to produce bulk quantities of metal nanoparticles.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 15/615,739, filed on Jun. 6, 2017, which is a divisional ofU.S. patent application Ser. No. 14/028,487, now U.S. Pat. No.9,700,940, filed on Sep. 16, 2013, which claims the benefit of priorityunder 35 U.S.C. Section 119 from U.S. Provisional Patent Application51/706,722, filed on Sep. 27, 2012, each of which is incorporated hereinby reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

FIELD OF THE INVENTION

The present invention generally relates to nanoparticles, and, morespecifically, to methods for producing metal nanoparticles in which thenanoparticles are grown around a nucleus that is derived from aplurality of in situ-generated nucleation seeds.

BACKGROUND

Although lead has traditionally been used in numerous industrialapplications, current regulations have mandated the elimination and/orphase out of lead in most commercial products. These mandates havestimulated new product development based upon lead-free technologies.

Soldering applications, particularly in electronics and vehiclemanufacturing, have been heavily impacted by the ban on lead. Numerousalternatives to traditional lead-based solders have been developed, theSn/Ag/Cu (SAC) system being among the most widely used, but many haveexhibited drawbacks that can make them unsuitable for use in certainapplications. For example, SAC solder can be unsuitable for extremeenvironments such as those found in automotive, military, and spacevehicles, where long life and reliability are of significant importance.Furthermore, SAC solder has a significantly higher eutectic meltingpoint (m.p. of ˜217° C.) than does traditional Sn/Pb solder (m.p. of183° C. for 63/37 Sn/Pb or 188° C. for 60/40 Sn/Pb), thus limiting itsuse to substrates that are capable of withstanding its relatively highprocessing temperature. The same is also true for many other lead-freesolder replacements. The need for high performance, thermally stablesubstrates for use in conjunction with SAC and other lead solderreplacements can significantly impact the cost of consumer productsrelative to those in which lower quality substrates can be used. Anotherlimitation of SAC solder is that its high tin content makes it prone totin whisker formation, which can increase the risk of electricalshorting.

Several compositions containing nanoparticles have also been proposed asreplacements for traditional lead-based solders. Metal nanoparticles,particularly those that are about 20 nm or less in size, can exhibit asignificant melting point depression over that of the corresponding bulkmetal, thereby allowing the nanoparticles to be liquefied attemperatures that are often comparable to those of traditionallead-based and lead-free solder materials. Copper nanoparticles, inparticular, have been extensively studied as an alternative soldermaterial. Although metal nanoparticles having a widely dispersed sizerange can be desirable in some instances, it can be more favorable insome applications for the metal nanoparticles to have a narrow, morecontrolled size range. Although some success has been realized in thisregard by using tailored combinations of surfactants during metalnanoparticle growth, scalable processes for reliably producing bulkquantities of metal nanoparticles in a targeted size range are not yetwell developed. Without being bound by any theory or mechanism, it isbelieved that in most metal nanoparticle syntheses, nanoparticlenucleation and growth are competing processes that take placeconcurrently. That is, as new metal nanoparticles are being formed by anucleation process, other nanoparticles continue to grow, therebyresulting in a wide nanoparticle size distribution.

In addition to soldering applications, metal nanoparticles have beenproposed for use in a number of other fields including, but not limitedto, communication, electronic, and medical uses. Production of bulkquantities of metal nanoparticles having a narrow and desired size rangeremains a challenge for implementing many of these contemplated uses ofmetal nanoparticles.

In view of the foregoing, scalable processes for producing metalnanoparticles that address current issues relating to nanoparticle sizedisparity would represent a substantial advance in the art. The presentinvention satisfies the foregoing need and provides related advantagesas well.

SUMMARY

In some embodiments, methods for producing metal nanoparticles aredescribed herein. In some embodiments, the methods include providing afirst metal salt in a solvent; converting the first metal salt into aninsoluble compound that constitutes a plurality of nanoparticle seeds;and after forming the plurality of nanoparticle seeds, reacting areducing agent with at least a portion of a second metal salt in thepresence of at least one surfactant and the plurality of nanoparticleseeds to form a plurality of metal nanoparticles. Each metalnanoparticle includes a metal shell formed around a nucleus derived froma nanoparticle seed, and the metal shell includes a metal from thesecond metal salt.

In some embodiments, methods for producing metal nanoparticles caninclude providing a first metal salt in a solvent, the first metal saltcontaining a copper (II) salt; reducing the first metal salt to form aninsoluble copper (I) compound that constitutes a plurality ofnanoparticle seeds; and after forming the plurality of nanoparticleseeds, reacting a reducing agent with a second metal salt in thepresence of at least one surfactant and the plurality of nanoparticleseeds to form a plurality of metal nanoparticles. Each metalnanoparticle includes a metal shell formed around a nucleus derived froma nanoparticle seed, and the metal shell includes a metal from thesecond metal salt.

In some embodiments, methods for producing copper nanoparticles caninclude providing a copper (II) salt in a solvent; reducing the copper(II) salt to an insoluble copper (I) salt having a particle size ofabout 10 nm or less; after forming the insoluble copper (I) salt, addingat least one surfactant and a reducing agent thereto; and reacting thereducing agent with the insoluble copper (I) salt in the presence of atleast one surfactant to form a plurality of copper nanoparticles.

In some embodiments, metal nanoparticle compositions are describedherein. In some embodiments metal nanoparticle compositions can includemetal nanoparticles containing a metal shell formed around a nucleusderived from a copper (I) nanoparticle seed, and a surfactant coatingdisposed on the metal shell, where the surfactant coating includes atleast one surfactant.

The foregoing has outlined rather broadly the features of the presentdisclosure in order that the detailed description that follows can bebetter understood. Additional features and advantages of the disclosurewill be described hereinafter. These and other advantages and featureswill become more apparent from the following description taken inconjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionsto be taken in conjunction with the accompanying drawings describingspecific embodiments of the disclosure, wherein:

FIG. 1 shows a presumed structure of a metal nanoparticle formed arounda nucleus derived from a nanoparticle seed;

FIG. 2 shows an illustrative STEM image on a gold TEM grid of copper (I)chloride nanoparticle seeds prepared via reduction of a copper (II)salt; and

FIGS. 3A and 3B show photographs of a 100 L reactor used in formingmetal nanoparticles at the nanoparticle seed stage (FIG. 3A) and afterforming metal nanoparticles (FIG. 3B).

DETAILED DESCRIPTION

The present disclosure is directed, in part, to methods for producingmetal nanoparticles having a nucleus derived from an in situ-generatednucleation seed, which may constitute an insoluble metal compound insome embodiments. The present disclosure is also directed, in part, tometal nanoparticles having a metal shell formed around a nucleus derivedfrom an in situ-generated nucleation seed, which may constitute aninsoluble metal compound in some embodiments. The present disclosure isalso directed, in part, to metal nanoparticles that are directlyproduced from an insoluble metal compound that is formed in situ andmethods for producing such metal nanoparticles. Other types ofnanoparticles can also be produced by extension of the present methods,as described hereinbelow.

Although some success has been realized in developing metal nanoparticlesyntheses that are capable of producing a narrow nanoparticle sizedistribution, such syntheses can sometimes lose their ability to controlthe nanoparticle size distribution at industrial scales (e.g., hundredsto thousands of liters and higher). This can particularly be the case insyntheses that use tailored surfactants to control metal nanoparticlenucleation and growth. Factors leading to loss of process control caninclude, for example, temperature fluctuations, mixing gradients,non-uniform nucleation site distribution, local supersaturation effects,and the like. Moreover, the procedures that do exist are not believed tobe generally applicable for forming any given type of metal nanoparticlein an intended size range.

In contrast to existing metal nanoparticle syntheses, the presentinventor discovered methods in which metal nanoparticle nucleation andgrowth processes can be simply and effectively separated from oneanother. By separating metal nanoparticle nucleation and growth from oneanother, metal nanoparticles having a narrow size distribution within adesired size range can be obtained much more readily, with much lessdependence on production scale. Specifically, the methods describedherein can produce good yields of metal nanoparticles that have a sizeof about 20-30 nm or below, particularly with a size of about 10 nm orbelow, or more particularly with a size of about 5 nm or below. Greatercontrol over the metal nanoparticle size distribution can, in turn,allow the physical and chemical properties of the metal nanoparticles tobe more readily tuned to meet the needs of a particular application inwhich the nanoparticles are being used. Moreover, the methods arereadily extendable to the formation of alloy nanoparticles andnon-metallic nanoparticles as well.

More particularly, the present inventor determined that seeded growth ofmetal nanoparticles can be used to accomplish the foregoing. Althoughseeded growth techniques can sometimes be successful for producinglarger particles, such as micron-scale particles, the problem is muchmore difficult in the nanoscale. In this regard, one has to first makethe seeds before nucleating the metal nanoparticles around them, therebyonly shifting the nucleation and growth problem to a different reactionthan that used to form the metal nanoparticles. Specifically, formingseeds for metal nanoparticle growth (also referred to herein as“nucleation seeds” or “nanoparticle seeds”) would ordinarily present thesame types of problems that arise with producing metal nanoparticlesthemselves (i.e., wide size distribution, isolation, and the like).Moreover, in most cases, the nanoparticle seeds are even smaller thanthe metal nanoparticles themselves. Still further, the separateformation of nanoparticle seeds would ordinarily increase the difficultyand cost associated with producing metal nanoparticles.

The present inventor recognized that certain insoluble salts and othersubstances stop growing very quickly following their formation from asoluble precursor, thereby producing a narrow distribution of very smallparticle sizes (e.g., below about 5 nm, particularly about 3 nm or belowin size down to molecular clusters that are only about 4-5 atoms insize). The formation of a narrow particle size distribution under theseconditions can take place essentially independent of reaction volume,because the insoluble particles form throughout the reaction volume andtheir limited solubility forces the rapid cessation of particle growth.Following their production, such insoluble substances can serve as aneffective template for growth of nanoparticles thereon (i.e., as ananoparticle seed). Specifically, in some embodiments, a metalnanoparticle having a nucleus derived from a nanoparticle seed can beformed by growing a metal shell around the nucleus. Other types ofnanoparticles, such as ceramic nanoparticles, can be grown in a likemanner around a nucleus derived from a pre-formed nanoparticle seed.Other illustrative nanoparticles that can be formed by growth from asuitable precursor around a nucleus include those disclosed in commonlyowned United States Application Publication 20110088739, which isincorporated herein by reference in its entirety. Since the seed-derivednucleus is buried within the metal shell and only constitutes a smallvolume fraction of the metal nanoparticle as a whole, the presence ofthe nucleus is not believed to appreciably impact the properties of themetal nanoparticles formed thereon. Thus, by separately producing aplurality of nanoparticle seeds with a narrow size distribution, metalnanoparticles can be produced therefrom in desired size range (e.g., bycontrolling the nanoparticle growth time) while maintaining a narrowsize distribution conferred by the nanoparticle seeds. Illustrativesubstances that can be readily formed and serve as nanoparticle seedsare discussed in more detail below.

In addition, the present inventor recognized that, in some embodiments,nanoparticle seed formation could readily take place in the samereaction vessel in which metal nanoparticle formation subsequently takesplace, thereby not significantly complicating the overall syntheticprocess. This feature can also limit the exposure of the nanoparticleseeds or the metal nanoparticles to air, which can be favorable in someembodiments. However, in some embodiments, optional separation of thenanoparticle seeds from their formation precursor can take place. Theuse of nanoparticle seeds in metal nanoparticle syntheses canadvantageously allow for metal nanoparticles to be formed from a metalthat is the same as or different than that which is present innanoparticle seeds. In addition, because the nanoparticle seedsconstitute but a small fraction of the overall volume of the metalnanoparticles, nanoparticle seeds containing non-metallic elements canbe used as well.

Without being bound by any theory or mechanism, FIG. 1 shows a presumedstructure of a metal nanoparticle formed around a nucleus derived from ananoparticle seed. Metal nanoparticle 20 includes metal shell 22 andsurfactant coating 24 disposed on metal shell 22. Metal nanoparticle 20also includes nucleus 26 located within metal shell 22. As describedabove, nucleus 26 can promote the deposition of metal shell 22 thereon.When nucleus 26 is incorporated within metal nanoparticle 20, it iseffectively shielded from its external environment (like the nucleus ofan atom) by metal shell 22, and it is not believed to contributeappreciably to the properties of metal nanoparticle 20 as a result.

Although FIG. 1 has depicted metal nanoparticle 20 as having adiscernible nucleus 26 surrounded by metal shell 22, this need notnecessarily be the case. For example, in the course of forming a metalnanoparticle, the nucleus can become reacted, alloyed, dissolved orotherwise admixed with the metal constituting the metal shell. Thenature of the nanoparticle seed and the chosen metal will determinewhether or not the nanoparticle seed remains as a discrete entity in themetal nanoparticles.

Moreover, the embodiments of the present disclosure are not necessarilylimited to the formation of metal nanoparticles around a nanoparticleseed. Other types of nanoparticles can be formed through variation ofthe methods described herein. Illustrative nanoparticles that can beformed through extension of the methods described herein include, forexample, alloy nanoparticles containing two or more metals, LaTe alloynanoparticles, periodic Group III/V nanoparticles, periodic Group II/VInanoparticles, carbide nanoparticles, nitride nanoparticles, oxidenanoparticles, boride nanoparticles, chalcogenide nanoparticles, andpnictide nanoparticles.

As used herein, the term “nanoparticles” refers to particles having asize of about 100 nm or less in equivalent spherical diameter, althoughnanoparticles need not necessarily be spherical in shape. As usedherein, the term “metal nanoparticles” refers to nanoparticles having ametal-containing shell formed around a nucleus, in which the shellcontains one or more metals, although the one or more metals need notnecessarily be in a zero-valent state. Reference to a particular type ofmetal nanoparticle will refer to the metal that is present in its metalshell. For example, the term “copper nanoparticles” refers to metalnanoparticles having a copper-containing shell formed around a nucleus.

As used herein, the term “size range” refers to the distribution ofnanoparticle sizes in a plurality of nanoparticles, such that >95% ofthe nanoparticles have a size residing within the indicated size range.

As used herein, the term “average size” refers to the arithmetic mean ofthe distribution of nanoparticle sizes in a plurality of nanoparticles.

As used herein, the term “maximum size” refers to the largestnanoparticle size in a plurality of nanoparticles.

As used herein, the term “fusion temperature” refers to the temperatureat which nanoparticles liquefy and fuse to one another, giving theappearance of melting. As used herein, the terms “fuse,” “fused,”“fusion,” and other variants thereof refer to a coalescence or partialcoalescence between two or more nanoparticles.

As used herein, the term “organic solvent” generally refers to polaraprotic organic solvents. Organic solvents of the embodiments describedherein are capable of solubilizing metal salts and/or reducing agents,or acting as co-solvents to solubilize metal salts and/or reducingagents. Suitable organic solvents can include, but are not limited to,alcohol and glycol solvents, which can also be used in combination withwater.

As used herein, the term “insoluble compound” refers to a substance thatforms as a precipitate, colloid, or sol from a soluble precursor in agiven solvent.

In some embodiments, methods described herein can include providing afirst metal salt in a solvent; converting the first metal salt into aninsoluble compound that constitutes a plurality of nanoparticle seeds;and after forming the plurality of nanoparticle seeds, reacting areducing agent with at least a portion of a second metal salt in thepresence of at least one surfactant and the plurality of nanoparticleseeds to form a plurality of metal nanoparticles. Each metalnanoparticle includes a metal shell formed around a nucleus derived froma nanoparticle seed, and the metal shell includes a metal from thesecond metal salt.

A wide variety of insoluble compounds are contemplated to be readilyformed from a soluble precursor in an appropriate solvent and utilizedin the formation of metal nanoparticles in accordance with theembodiments described herein. Illustrative examples of suitableinsoluble compounds and techniques for their formation are described inmore detail hereinafter. As will be recognized by one having ordinaryskill in the art, the solubility of a particular compound will often bedictated by the solvent in which a soluble precursor to the insolublecompound is present. By knowing the general solubility of compounds inparticular solvents, one of ordinary skill in the art will be able tochoose a suitable solvent from which to form an insoluble compound thatconstitutes a plurality of nanoparticle seeds and also from which tosubsequently grow metal nanoparticles therefrom.

In some embodiments, the insoluble compound can be formed by reducingthe first metal salt to form an insoluble compound. Some measure of sizecontrol of the nanoparticle seeds in the insoluble compound can berealized by modulating the reduction temperature and the length of timethe reaction mixture is stirred following the initial reduction,although the size distribution is thought to be largely due to theinnate solubility of the nanoparticle seeds in the chosen solvent. Inmore particular embodiments, the first metal salt can be reduced by afirst portion of the second metal salt to form the insoluble compound.Specifically, in some embodiments, the methods described herein canemploy an internal oxidation-reduction reaction that takes place betweenthe first metal salt and the second metal salt to form the insolublecompound. In such embodiments, the second metal salt is oxidizable andthe first metal salt is reduced by the second metal salt to form theinsoluble compound. The insoluble compound can be a zero-valent metal oran insoluble metal salt that contains a metal from the first metal salt.The insoluble compound can constitute a plurality of nanoparticle seedsupon which metal nanoparticles can be grown subsequently. In someembodiments, the metal nanoparticles can be grown by reacting theremaining second metal salt and/or an oxidation product of the secondmetal salt with a reducing agent in the presence of at least onesurfactant, or a different metal salt entirety (i.e., a third metalsalt) can be reacted with the reducing agent to form the metalnanoparticles.

In other embodiments, the first metal salt can be reduced by a thirdmetal salt to form the insoluble compound. In such embodiments, themetal nanoparticles can again be formed from the second metal salt. Insome embodiments, the metal from the third metal salt does not becomeincorporated in the metal nanoparticles. Keeping the metal from thethird metal salt from becoming incorporated in the metal nanoparticlescan be accomplished by choosing the third metal salt so that it does notbecome reduced to a zero-valent metal for forming a metal shell, or byseparating the insoluble compound from the third metal salt or itsoxidation product before forming the metal nanoparticles. In someembodiments, the methods described herein can further include separatingthe insoluble compound from the third metal salt or an oxidation productthereof, prior to reacting the reducing agent with the second metalsalt. It is to be recognized that in other embodiments described herein,it can also sometimes be desirable to separate the insoluble compoundprior to forming metal nanoparticles therefrom.

In still other embodiments, the first metal salt can be reduced by asugar containing an oxidizable group in order to form the insolublecompound. That is, in such embodiments, the first metal salt can bereduced with a reducing sugar to form the insoluble compound. Thestructure of reducing sugars and suitable examples thereof will befamiliar to one having ordinary skill in the art.

In some embodiments, at least the first metal salt can be a copper (II)salt, and the insoluble compound can be a copper (I) salt. Copper (II)salts are soluble in a wide range of solvents. Suitable copper (II)salts are not believed to be particularly limited and can include, forexample, copper (II) chloride, copper (II) sulfate, copper (II) acetate,copper (II) nitrate, and the like. Oxidizable metal salts, reducingsugars and other mild reducing agents readily reduce copper (II) to formcopper (I). Unlike copper (II) salts, copper (I) salts are extremelyinsoluble in all common solvents, and the copper (I) salts canprecipitate as a very finely divided insoluble compound that canconstitute a plurality of nanoparticle seeds. The reductant that broughtabout the formation of the copper (I) salt can, in contrast, remainsoluble in the solvent. In some embodiments, the first metal salt can bea copper (II) salt, and the second metal salt can be a metal salt thatcontains a different metal. In such embodiments, the methods describedherein can produce hybrid metal nanoparticles having a nucleus thatcontains copper and a metal shell that contains another metal. Asdescribed above, such hybrid metal nanoparticles can manifest only theproperties of the metal shell, as if the nucleus was not present. Inother embodiments, both the first metal salt and the second metal saltcan be a copper (II) salt. Accordingly, in such embodiments, the metalshell and the nucleus can both contain the same metal, copper.

In addition to the copper nanoparticles described above, otherillustrative metal nanoparticles that can be formed using an insolublecopper (I) salt as a plurality of nanoparticle seeds are tinnanoparticles. Reactions that can be associated with forming tinnanoparticles in this manner are summarized in Formulas (1-3) below. InFormula (3), designation of the Cu_(nano) in parentheses indicates thatit is located as the nucleus of the tin nanoparticles so obtained. Theinitially formed Cu⁺ _(nano) can be reduced in situ to copper (0) duringreduction to form the tin nanoparticles around a copper nucleus. Othermetal nanoparticles can be formed in a like manner, as discussed in moredetail below.

Sn²⁺→Sn⁴⁺+2e⁻  Formula (1)

2Cu²⁺+2e⁻→2Cu⁺ _(nano)   Formula (2)

Sn²⁺/Sn⁴⁺+reducing agent+Cu⁺ _(nano)→Sn_(nano) (Cu_(nano))  Formula (3)

FIG. 2 shows an illustrative STEM image on a gold TEM grid of copper (I)chloride nanoparticle seeds prepared via reduction of a copper (II)salt. The copper (I) chloride nanoparticle seeds range in size from 3-6nm. FIG. 3A shows a photograph of a 100 L reactor used in forming metalnanoparticles at the nanoparticle seed stage, and FIG. 3B shows thedifferent appearance of the reactor contents after forming metalnanoparticles.

Other examples of oxidizable metal salts that can be used as the secondmetal salt for purposes of reducing the first metal salt to form aninsoluble compound can include, for example, Fe²⁺ salts, Ti³⁺ salts,Ce³⁺ salts, and the like. The reactions of these ions as their chloridesalts are set forth in Formulas (4)-(6). The residual second metal salt(e.g., in the form of Fe²⁺/Fe³⁺, Ti³⁺/Ti⁴⁺ or Ce³⁺/Ce⁴⁺) cansubsequently be converted to iron nanoparticles, titanium nanoparticlesor cerium nanoparticles, respectively, each having a nucleus derivedfrom an insoluble copper (I) salt. These oxidizable metal salts andothers can also be used as a third metal salt in a like manner to thatdescribed above.

CuCl₂+FeCl₂→FeCl₃+CuCl_(nano)   (Formula 4)

CuCl₂+TiCl₃→TiCl₄+CuCl_(nano)   (Formula 5)

CuCl₂+CeCl₃→CeCl₄+CuCl_(nano)   (Formula 6)

Other oxidizable metal salts that are capable of reducing copper (II) tocopper (I) can be used in a like manner as the second metal salt. One ofordinary skill in the art will be able to determine suitable examples ofsuch oxidizable metal salts through having the benefit of thisdisclosure and by referring to a table of standard oxidation/reductionpotentials and the known solubility properties of the metal salts.

In some embodiments, nanoparticle seeds containing copper (I) can alsobe generated from basic copper (II) solutions that are stabilized withammonia, citrate, or tartrate, for example. The addition of a reducingsugar or other readily oxidizable compound, particularly one thatcontains one or more aldehyde functional groups, can reduce copper (II)to insoluble Cu₂O that precipitates as a very fine yellow material thatthen turns red. The Cu₂O so formed can be used in the formation of metalnanoparticles in a like manner to that described above for CuCl andother insoluble copper (I) salts. The reaction to produce Cu₂O in theforegoing manner is shown in Formula 7.

2Cu(OH)₂+C₆H₁₂O₆→Cu₂O_(nano)+C₆H₁₂O₇+2H₂O  (Formula 7)

In the foregoing embodiments, the first metal salt is not limited tojust a copper (II) salt. In general, any first metal salt that forms ahighly insoluble compound upon reduction can be used in variousembodiments of the present disclosure. Other first metal salts that canform insoluble compounds following reduction (e.g., with tin (II))include, for example, iron (III) salts. For example, iron (II)nanoparticle seeds can be produced by reduction of an iron (III) salt ina solvent like triglyme.

The first metal salt also need not necessarily be reduced by a secondmetal salt in order to form an insoluble compound. As described above,in some embodiments, a reducing sugar can be used to convert the firstmetal salt into the insoluble compound. Likewise, in other alternativeembodiments, the first metal salt can be reduced using an organiccompound containing an oxidizable group or using an inorganic reducingagent. These types of compounds can remain soluble in the solvent afterforming the insoluble compound and subsequent reduction of the secondmetal salt to form the metal nanoparticles. In still other alternativeembodiments, the first metal salt can be reduced with a third metalsalt. In some embodiments, the third metal salt can be reacted with thefirst metal salt to form the insoluble compound in the absence of thesecond metal salt. The insoluble compound can then be separated from thethird metal salt or its oxidation product and subsequently combined withthe second metal salt, which can then be reacted with a reducing agentto form the metal nanoparticles. Alternative strategies for forming theinsoluble compound from the first metal salt through routes other thanreduction are discussed in more detail below.

In some embodiments, nanoparticle seeds formed through reduction of thefirst metal salt can have a metal oxidation state of +1 or above afterbeing reduced, particularly through reduction using a second metal saltor a third metal salt. As discussed above, the reduction of copper (II)to copper (I) represents an illustrative example. In other embodiments,the nanoparticle seeds can have a metal oxidation state of 0 after beingreduced. Illustrative metal ions that can be reduced to zero-valentnanoparticle seeds in the foregoing manner include, for example, goldand silver ions, for example. Suitable silver (I) and gold (III) saltsthat can be used to form zero-valent nanoparticle seeds include, forexample, silver (I) nitrate and gold (III) chloride. Auric acid (HAuCl₄)can also be used in a similar manner. Compared to copper salts, however,the latter metals are extremely expensive and not well suited for largescale production of metal nanoparticles as a result.

In alternative embodiments, the first metal salt need not be reduced, bya second metal salt or otherwise, in order to form an insoluble compoundthat can constitute a plurality of nanoparticle seeds. A number ofapproaches can be used to rapidly form insoluble compounds from a firstmetal salt, whose particles stop growing rapidly and accordingly have anarrow particle size distribution. Several of these approaches forforming nanoparticle seeds are discussed below.

In some embodiments, a soluble first metal salt can be reacted with anagent that results in precipitation of an insoluble salt containing themetal cation of the first metal salt. As referenced above, one ofordinary skill in the art will be able to determine the solubility of aparticular metal salt by referencing a table of known metal saltsolubility properties. A number of metal salts of organic acids (i.e.,metal carboxylates) are insoluble. Illustrative examples of metalcarboxylates that can be suitable as nanoparticle seeds include, forexample, metal benzoates and metal oxalates. For example, highlyinsoluble silver benzoate can be formed by reacting silver nitrate withbenzoic acid, and calcium oxalate can be formed by reacting calciumchloride with oxalic acid. In other embodiments, the first metal saltcan be reacted to produce a metal sulfide. One of ordinary skill in theart will recognize that many metal sulfides, particularly transitionmetal sulfides, are highly insoluble and can be prepared as finelydivided precipitates by reacting a soluble salt with hydrogen sulfide ora hydrogen sulfide-generating compound.

In still other embodiments, a soluble first metal salt can be reactedwith a ligand that results in precipitation of an insoluble metal-ligandcomplex containing the metal cation of the first metal salt. Forexample, in some embodiments, a metal-salen complex can be used as theinsoluble compound constituting the nanoparticle seeds(salen=condensation product of salicylaldehyde and ethylenediamine, or aderivative of either of these compounds). One of ordinary skill in theart will recognize that many salen complexes are highly insoluble andform finely divided precipitates. Insoluble metal complexes can also beformed from other ligands or combinations of ligands and can also beused in other embodiments of the present disclosure.

In still other alternative embodiments, sols can be used to form theinsoluble compound of the plurality of nanoparticle seeds. Suitable solscan include, for example, silica sols and tin (IV) oxide sols. Althoughsilica sols are typically formed from covalent silicon tetraalkoxycompounds, for the purposes of this disclosure, these types of compoundswill be considered to constitute a metal salt. Hydroxide sols can alsobe used in some embodiments described herein. An advantage of certainhydroxide sols is that as they age, their particle size changes, therebyoffering some degree of tunability to the nanoparticle seed size.Suitable hydroxide sols can include, for example, Fe(OH)₃, Al(OH)₃, andMo/W hydroxide sols. In still other embodiments, insoluble sols can beformed from bidentate ligands that cannot bond to a single metal centerdue to steric hindrance, such that bridging structures are insteadformed.

In some embodiments, the insoluble compound constituting the pluralityof nanoparticle seeds can constitute colloidal sulfur. Colloidal sulfurcan be formed by the metathesis reaction between sodium sulfide andstrontium chloride in water or methanol as a solvent, for example. Instill further alternative embodiments, periodic Group II carbonates orsulfates, for example, can be precipitated from water as insolublenanoparticle seeds. For example, in some embodiments, CaCO₃ or CaSO₄ canbe precipitated from water as a very fine suspension having a milkyappearance. Similarly, in some embodiments, HgI₂, CdI₂, or BiCl₃ can bereacted with Na₂Te to form the respective tellurides or selenides as theinsoluble nanoparticle seeds. Concentration and temperature can be usedto control the size of the nanoparticle seeds to some degree.

Accordingly, in view of the foregoing, it is believed that metalnanoparticles having any metal in the metal shell formed around thenucleus can be produced through appropriate modifications of the methodsdescribed herein. In this regard, illustrative metals that can bepresent in the metal shell produced from the second metal salt andformed by the methods described herein include transition metals (Sc,Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd,Hf, Ta, W, Re, Os, Ir, Pt, Au), main group metals (e.g., Al, Ga, In, Tl,Sn, Pb, Sb, Bi), and lanthanide metals (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb,Dy, Ho, Er, Tm, Yb, Lu). In some embodiments, the second metal salt canconstitute a mixture of metal salts of any two or more of the foregoingmetals, such that the metal shell is an alloy and alloy nanoparticlesare formed. Any salt form of these elements that is soluble in thechosen solvent can be used as the second metal salt for purposes offorming the metal shell of the metal nanoparticles. Moreover, the metalthat is present in the metal shell of the metal nanoparticles can be thesame as or different than that which is present in the nucleus derivedfrom the nanoparticle seed. That is, in some embodiments, the firstmetal salt and the second metal salt can be the same, and in otherembodiments, the first metal salt and the second metal salt can bedifferent. Further, in view of the above description regarding thenanoparticle seeds, it is to be recognized that a metal shell can beformed around a nanoparticle seed that is non-metallic but is formedfrom a first metal salt (e.g., colloidal sulfur).

In more particular embodiments, methods described herein can includeproviding a first metal salt in a solvent, the first metal saltincluding a copper (II) salt; reducing the first metal salt to form aninsoluble copper (I) compound, the insoluble copper (I) compoundconstituting a plurality of nanoparticle seeds; and after forming theplurality of nanoparticle seeds, reacting a reducing agent with a secondmetal salt in the presence of at least one surfactant and the pluralityof nanoparticle seeds to form a plurality of metal nanoparticles, eachmetal nanoparticle containing a metal shell formed around a nucleusderived from a nanoparticle seed, and in which the metal shell containsa metal from the second metal salt.

In some embodiments, the first metal salt containing the copper (II)salt can be reduced by a first portion of the second metal salt. In suchembodiments, the first metal salt and the second metal salt aredifferent. Accordingly, in such embodiments, the metal nanoparticles canhave a metal shell that contains a different metal than the copper whichis present in the nucleus of the metal nanoparticles.

In other embodiments, the first metal salt containing the copper (II)salt can be reduced by a third metal salt. In some embodiments, themethods can further include separating the insoluble copper (I) compoundfrom the third metal salt or an oxidation product thereof, prior toreacting the reducing agent with the second metal salt to form theplurality of metal nanoparticles. In some embodiments, the methods canfurther include combining the second metal salt with the nanoparticleseeds, after separating the nanoparticle seeds from the third metal saltor an oxidation product thereof. In such embodiments, the metal shellcan contain a metal that is the same as or different than the copperthat is present in the nucleus of the metal nanoparticles. That is, insome embodiments, the second metal salt can be a copper (II) salt.

Since the nucleus around which the metal nanoparticles are formedconstitutes but a small percentage of the overall nanoparticle volume,the first metal salt is generally present in a lesser amount relative tothe second metal salt. By adjusting the amount of the second metal saltthat is present and other factors, the thickness of the metal shell ofthe metal nanoparticles can be controlled, thereby dictating the overallsize of the metal nanoparticles, since the nucleus derived from thenanoparticle seeds is relatively invariant in size. In some embodiments,the first metal salt is present in an amount ranging between about 0.01and about 0.5 stoichiometric equivalents relative to the second metalsalt. In other embodiments, the first metal salt is present in an amountranging between about 0.05 and about 0.25 stoichiometric equivalentsrelative to the second metal salt, or in an amount ranging between about0.05 and about 0.1 stoichiometric equivalents relative to the secondmetal salt, or in an amount ranging between about 0.1 and about 0.15stoichiometric equivalents relative to the second metal salt, or in anamount ranging between about 0.15 and about 0.2 stoichiometricequivalents relative to the second metal salt, or in an amount rangingbetween about 0.2 and about 0.25 stoichiometric equivalents relative tothe second metal salt. In more specific embodiments, a stoichiometricratio of the first metal salt to the second metal salt can be about0.01, or about 0.02, or about 0.03, or about 0.04, or about 0.05, orabout 0.06, or about 0.07, or about 0.08, or about 0.09, or about 0.1,or about 0.11, or about 0.12, or about 0.13, or about 0.14, or about0.15, or about 0.16, or about 0.17, or about 0.18, or about 0.19, orabout 0.20, or about 0.21, or about 0.22, or about 0.23, or about 0.24,or about 0.25.

In some embodiments, metal nanoparticles produced by the methodsdescribed herein can range from about 1 nm to about 20-30 nm in size. Insome or other embodiments, the metal nanoparticles can range betweenabout 1 nm to about 5 nm in size, or between about 1 nm to about 10 nmin size, or between about 5 nm to about 10 nm in size, or between about10 nm to about 15 nm in size, or between about 15 nm to about 20 nm insize, or between about 20 nm to about 25 nm in size, or any subrange orcombination of these ranges. In some embodiments, the metalnanoparticles can have a size distribution ranging between about 3 nmand about 5 nm, which is close to monodispersity.

As discussed above, metal nanoparticles formed by the methods describedherein can have a surfactant coating disposed on the metal shell formedaround the nucleus, in which the surfactant coating contains at leastone surfactant. The at least one surfactant can be physically bonded tothe metal shell, chemically bonded to the metal shell or any combinationthereof. The inclusion of one or more surfactants during the synthesisof the metal nanoparticles can result in their stabilization and evenfurther limit growth of metal nanoparticles to a desired size. Amongother features, stabilization of the metal nanoparticles using one ormore surfactants can include, for example, preventing or limitingagglomeration of the metal nanoparticles, controlling the size of themetal nanoparticles, protecting the surface of the metal nanoparticlesfrom oxidation, or any combination thereof.

In some embodiments, one surfactant can be included in the synthesis ofthe metal nanoparticles and become disposed on the metal shell. In otherembodiments, two or more surfactants can be included in the synthesis ofthe metal nanoparticles and become disposed on the metal shell. In someembodiments, three or more surfactants can be included in the synthesisof the metal nanoparticles and become disposed on the metal shell. Byusing mixtures of surfactants, the properties of the metal nanoparticlescan be further tailored. For example, some surfactants will interactmore strongly with certain metals than with others, thereby requiringmore vigorous conditions, such as heating, to remove the surfactant(s)from the metal nanoparticles. Further discussion of suitable surfactantsand benefits of using mixtures of surfactants in combination with oneanother will be discussed hereinbelow. A brief discussion ofillustrative surfactants that interact strongly with particular metalswill also be provided below.

In some embodiments, a combination of amine surfactants, particularlyaliphatic amines, can be used during the synthesis of metalnanoparticles and become disposed on the metal shell of the metalnanoparticles. In some embodiments, two amine surfactants can be used incombination with one another. In other embodiments, three aminesurfactants can be used in combination with one another. In morespecific embodiments, a primary amine and a diamine chelating agent canbe used in combination with one another, or a primary amine, a secondaryamine, and a diamine chelating agent can be used in combination with oneanother. In still more specific embodiments, the three amine surfactantscan include a long chain primary amine, a secondary amine, and a diaminehaving at least one tertiary alkyl group nitrogen substituent. In someor other embodiments, triamines, tetraamines, orethylenediaminetetraacetic acid can be used as an amine surfactantduring the formation of metal nanoparticles, optionally in combinationwith the other surfactants described herein. Such multidentatesurfactants can be desirable for forming three-dimensional networks,such as sols, in some embodiments. Azodicarboxamides can also be used ina similar manner in some embodiments. Further disclosure regardingsuitable amine surfactants follows hereinafter.

In some embodiments, the at least one surfactant can include a primaryalkylamine. In some embodiments, the primary alkylamine can be a C4-C18alkylamine, or a C6-C18 primary alkylamine. In some embodiments, theprimary alkylamine can be a C7-C10 alkylamine. In other embodiments, aC5-C6 primary alkylamine can also be used. Without being bound by anytheory or mechanism, the exact size of the primary alkylamine can bebalanced between being long enough to provide an effective inversemicelle structure versus having ready volatility and/or ease ofhandling. For example, primary alkylamines with more than 18 carbons canalso be suitable for use in the present embodiments, but they can bemore difficult to handle because of their waxy character. C7-C10 primaryalkylamines, in particular, can represent a good balance of desiredproperties for ease of use.

In some embodiments, the C4-C18 primary alkylamine can be n-heptylamine,n-octylamine, n-nonylamine, or n-decylamine, for example. While theseare all straight chain primary alkylamines, branched chain primaryalkylamines can also be used in other embodiments. For example, branchedchain primary alkylamines such as, for example, 7-methyloctylamine,2-methyloctylamine, or 7-methylnonylamine can be used. In someembodiments, such branched chain primary alkylamines can be stericallyhindered where they are attached to the amine nitrogen atom.Non-limiting examples of such sterically hindered primary alkylaminescan include, for example, t-octylamine, 2-methylpentan-2-amine,2-methylhexan-2-amine, 2-methylheptan-2-amine, 3-ethyloctan-3-amine,3-ethylheptan-3-amine, 3-ethylhexan-3-amine, and the like. Additionalbranching can also be present. Without being bound by any theory ormechanism, it is believed that primary alkylamines can serve as ligandsin the coordination sphere of the metal shell. Due to their single pointof attachment, they are believed to be readily dissociable from themetal shell, particularly under heating conditions. For example, duringmetal nanoparticle fusion, the primary alkyl amines and/or othersurfactants can dissociate from the metal shell as the nanoparticlesbecome fused together. For primary alkylamines having significant sterichinderance, the bulky groups near the amine nitrogen atom can furtherreduce the bonding strength and facilitate dissociation from the metalshell.

In some embodiments, the at least one surfactant can include a secondaryamine, particularly a secondary aliphatic amine. Secondary aminessuitable for use in the present embodiments can include normal,branched, or cyclic C4-C12 alkyl groups bound to the amine nitrogenatom. In some embodiments, the branching can occur on a carbon atombound to the amine nitrogen atom, thereby producing significant stericencumbrance at the nitrogen atom. Secondary amines that can be used inthe synthesis of metal nanoparticles and can become disposed on themetal shell include, without limitation, dihexylamine, diisobutylamine,di-t-butylamine, dineopentylamine, di-t-pentylamine, dicyclopentylamine,dicycylohexylamine, and the like. Secondary amines outside the C4-C12range can also be used, but such secondary amines can have undesirablephysical properties such as low boiling points or waxy consistenciesthat complicate their handling. Without being bound by any theory ormechanism, it is believed that secondary amines can more stronglycoordinate to the metal shell due to their higher basicity, although thebonding strength can be tempered when the secondary amine is stericallyencumbered. Remaining unbound by any theory or mechanism, it is believedthat secondary amines can particularly facilitate the dissolution ofmetal salts in an organic solvent.

In some embodiments, the at least one surfactant can include a chelatingagent, particularly a diamine chelating agent. In some embodiments, oneor both of the nitrogen atoms of the diamine chelating agent can besubstituted with one or two alkyl groups each. When two alkyl groups arepresent on the same nitrogen atom, they can be the same or different.Further, when both nitrogen atoms are substituted, the same or differentalkyl groups can be present. In some embodiments, the alkyl groups canbe C1-C6 alkyl groups. In other embodiments, the alkyl groups can beC1-C4 alkyl groups or C3-C6 alkyl groups. In some embodiments, C3 orhigher alkyl groups can be straight or have branched chains. In someembodiments, C3 or higher alkyl groups can be cyclic. Without beingbound by any theory or mechanism, it is believed that diamine chelatingagents can coordinate a metal center at two locations and stabilize theformation of metal nanoparticles. Formation of a metal chelate canresult in the diamine chelating agent being more strongly bound to themetal shell than are non-chelated surfactants. It is to be recognized,however, that a diamine chelating agent also bridge between two metalnanoparticles in some embodiments.

In some embodiments, suitable diamine chelating agents can includeN,N′-dialkylethylenediamines, particularly C1-C4N,N′-dialkylethylenediamines. The corresponding methylenediarnine,propylenediamine, butylenediamine, pentylenediamine or hexylenediamineanalogues can also be used. The alkyl groups can be the same ordifferent. C1 -C4 alkyl groups that can be present include, for example,methyl, ethyl, propyl, and butyl groups, or branched alkyl groups suchas isopropyl, isobutyl, s-butyl, and t-butyl groups. IllustrativeN,N′-dialkylethylenediamines that can be suitable for use in the presentembodiments include, for example, N,N′-dimethylethylenediamine,N,N′-diethylethylenediamine, N,N′-di-t-butylethylenediamine,N,N′-diisopropylethylenediamine, and the like.

In some embodiments, suitable diamine chelating agents can includeN,N,N′,N′-tetraalkylethylenediamines, particularly C1-C4N,N,N′,N′-tetraalkylethylenediamines. The correspondingmethylenediamine, propylenediamine, butylenediamine, pentylenediamine orhexylenediamine analogues can also be used. The alkyl groups can againbe the same or different and include those mentioned above. IllustrativeN,N,N′,N′-tetraalkylethylenediamines that can be suitable for use in thepresent embodiments include, for example,N,N,N′,N′-tetramethylethylenediamine,N,N,N′,N′-tetraethylethylenediamine, and the like.

Surfactants other than amines, particularly aliphatic amines, can alsobe used in the embodiments described herein. Illustrative examples ofsuitable surfactants that can be used in this regard include, forexample, pyridines, aromatic amines, phosphines, thiols, and anycombination thereof. These surfactants can be used in combination withan aliphatic amine, including those described above, or they can be usedin a surfactant system in which an aliphatic amine is not present.

Suitable aromatic amines can have a formula of ArNR¹R², where Ar is asubstituted or unsubstituted aryl group and R¹ and R² are the same ordifferent. R¹ and R² can be independently selected from H or an alkyl oraryl group containing from 1 to about 16 carbon atoms. Illustrativearomatic amines that can be suitable for use in the present embodimentsinclude, for example, aniline, toluidine, anisidine,N,N-dimethylaniline, N,N-dimethylaniline, and the like. Other aromaticamines that can be used in conjunction with the embodiments describedherein can be envisioned by one having ordinary skill in the art.

Suitable pyridines include both pyridine and its derivatives.Illustrative pyridines that can be suitable for use in the presentembodiments include, for example, pyridine, 2-methylpyridine,2,6-dimethylpyridine, collidine, pyridazine, and the like. Otherpyridines that can be used in conjunction with the embodiments describedherein can be envisioned by one having ordinary skill in the art.

Suitable phosphines can have a formula of PR₃, where R is an alkyl oraryl group containing from 1 to about 16 carbon atoms. The alkyl or arylgroups attached to the phosphorus center can be the same or different.Illustrative phosphines that can be used in the present embodimentsinclude, for example, trimethylphosphine, triethylphosphine,tributylphophine, tri-t-butylphosphine, trioctylphosphine,triphenylphosphine, and the like. Phosphine oxides can also be used in alike manner. In some embodiments, surfactants that contain two or morephosphine groups configured for forming a chelate ring can also be used.Illustrative chelating phosphines can include 1,2-bisphosphines,1,3-bisphosphines, and bis-phosphines such as BINAP, for example. Otherphosphines that can be used in conjunction with the embodimentsdescribed herein can be envisioned by one having ordinary skill in theart.

Suitable thiols can have a formula of RSH, where R is an alkyl or arylgroup having from about 4 to about 16 carbon atoms. Illustrative thiolsthat can be used in the present embodiments include, for example,butanethiol, 2-methyl-2-propanethiol, hexanethiol, octanethiol,benzenethiol, and the like. In some embodiments, surfactants thatcontain two or more thiol groups configured for forming a chelate ringcan also be used. Illustrative chelating thiols can include, forexample, 1,2-dithiols (e.g., 1,2-ethanethiol) and 1,3-dithiols (e.g.,1,3-propanethiol). Other thiols that can be used in conjunction with theembodiments described herein can be envisioned by one having ordinaryskill in the art.

As described above, the nature of the metal that is present in the metalshell of the metal nanoparticles and how strongly it is desired that thesurfactant is held to the metal shell can help determine whether asurfactant is suitable for use with a particular type of metalnanoparticles. For example, amine surfactants, including those notedabove, associate strongly with transition metals and can be particularlyadvantageous in this regard. One or more amine surfactants may beparticularly beneficial for the synthesis of copper nanoparticles, forexample. In contrast, amine surfactants generally bind much lessstrongly to main group metals. Without being bound by any theory ormechanism, it is believed that main group metals are “softer” and aremore strongly bonded by surfactants that prefer softer metal centers. Inthis regard, pyridines, aromatic amines, phosphines, and thiols can bemore suitable surfactants for softer metal centers. One or more of thesetypes of surfactants can be particularly beneficial in the synthesis oftin nanoparticles, for example. As described above, one or morealiphatic amines, including any aliphatic amine described above, can beused in combination with pyridines, aromatic amines, phosphines, andthiols to help further modulate the properties of the metalnanoparticles. Further, for making metal oxide nanoparticles, alcoholand carboxylic acid surfactants can be desirable.

In some embodiments, the at least one surfactant can include a firstsurfactant that contains a C6-C18 primary alkyamine and a secondsurfactant that contains a N,N′-dialkylethylenediamine, particularly aC1-C4 N,N′-dialkylethylenediamine. In further embodiments the at leastone surfactant can further include a third surfactant that contains asecondary alkylamine, particularly a C4-C12 secondary alkylamine.

Reducing agents that can be reacted with the second metal salt in orderto promote the formation of metal nanoparticles are not believed to beparticularly limited, other than having the capability for reducing thesecond metal salt to form a zero-valent metal. In some embodiments, thereducing agent can be an alkali metal such as, for example, lithium,sodium, or potassium, in the presence of a suitable catalyst. In someembodiments, the reducing agent can be lithium naphthalide, sodiumnaphthalide, or potassium naphthalide. In other embodiments, thereducing agent can be sodium borohydride, lithium borohydride, atetraalkylammonium borohydride, or a like borohydride reducing agent. Instill other embodiments, the reducing agent can be ascorbic acid, citricacid, a hydroxylamine, a reducing sugar, an aldehyde, or NaH₂PO₂, forexample. Additional reducing agents suitable for use in conjunction withforming metal nanoparticles can be envisioned by one having ordinaryskill in the art.

In general, metal nanoparticles can be formed in the presence of a smallexcess of the reducing agent, typically about a 5% molar excess or aboverelative to the second metal salt, or about a 10% molar excess or above.Larger molar excesses of up to about a 100% molar excess can also beused, if desired. In some embodiments, up to about a 25% molar excess ofthe reducing agent can be used relative to the second metal salt. Loweramounts of the reducing agent can be used if the reaction mixture ismaintained under dry conditions so as not to decompose excessive amountsof the reducing agent, particularly for those reducing agents that canbe reactive with atmospheric moisture or admixed water.

In addition to the second metal salt, the nanoparticle seeds can, insome embodiments, be reduced by the reducing agent in concert withforming the metal shell around the nucleus. Thus, the nucleus of thenanoparticles formed by the methods described herein can, in someembodiments, also be metallic like the metal shell. However, in otherembodiments, the nucleus can maintain a metal oxidation state of +1 orabove. In the case of copper (I) nanoparticle seeds, the copper (I) isreadily reduced, such that the nucleus becomes at least partiallyreduced to zero-valent copper upon reducing the second metal salt toform the metal shell around the nucleus. Reducing the nucleus to form azero-valent metal in this manner advantageously avoids the introductionof significant anions (e.g., chloride anions) from the nanoparticleseeds into the metal nanoparticles. As described above, in someembodiments, the nanoparticle seeds can also become reacted, alloyed, orotherwise admixed with the metal shell in the course of forming themetal nanoparticles.

In some embodiments, the metal nanoparticles can be formed in a solventthat is substantially anhydrous (e.g., about 200 ppm water or lower).Suitable techniques for drying solvents will be familiar to one havingordinary skill in the art. Suitable solvents include those set forthhereinbelow.

In some embodiments, the nanoparticle seeds can be formed in a firstsolvent, and the reducing agent used to form the metal nanoparticles canbe present in a second solvent, with the first solvent and the secondsolvent being combined to result in the formation of the metalnanoparticles. The first solvent and the second solvent in suchembodiments can be the same or different. In some embodiments, thenanoparticle seeds and the metal particles can all be formed in the samereaction vessel.

In some embodiments, the solvent in which the metal nanoparticles areformed can be an organic solvent, particularly any polar aprotic solventthat is capable of at least partially solubilizing one or more metalsalts and a reducing agent. As described above, in some embodiments, theorganic solvent can be substantially anhydrous. Further, in someembodiments, the organic solvent can be substantially oxygen free.Suitable organic solvents for solubilizing metal salts and forming metalnanoparticles can include, for example, formamide,N,N-dimethylformamide, dimethyl sulfoxide, dimethylpropylene urea,hexamethylphosphoramide, tetrahydrofuran, and methyl or ethyl ethers ofglyme, diglyme, triglyme, and tetraglyme. Diglyme or triglyme can beparticularly advantageous, since they readily dissolve a number of metalsalts while also activating reducing agents such as sodium borohydride.In some embodiments, the metal salt can be dissolved in a first solvent,and the reducing agent can be dissolved in triglyme, which is thencombined with the first solvent. Approaches in which triglyme is used asthe solvent can have a reducing agent concentration ranging betweenabout 0.5 M and about 3 M, allowing small reaction volumes to behandled. In alternative embodiments, the solvent can include water as acomponent in an organic solvent, or water alone can be used as thesolvent. Particularly, in some embodiments, the nanoparticle seeds canbe formed from a water-based solvent.

In some embodiments, any of the metal salts used in the methodsdescribed herein can be dried by techniques such as, for example,heating under vacuum or reacting with a chemical drying agent such as,for example, an orthoester or thionyl chloride. Drying of the metalsalts can help maintain the solvent in a substantially dry state in theevent that an anhydrous solvent is used.

In some embodiments, formation of the metal nanoparticles can take placein a reduction that occurs at a temperature below about 100° C., orbelow about 80° C., or below about 50° C. In some embodiments, formationof the metal nanoparticles can take place at a temperature rangingbetween about 0° C. and about 50° C., or between about 25° C. and about50° C., or between about 25° C. and about 80° C., or between about 30°C. and about 40° C. As one of ordinary skill in the art will appreciate,higher temperatures will generally facilitate the faster formation ofmetal nanoparticles. Further, one of ordinary skill in the art willappreciate that the size of the metal nanoparticles obtained may depend,at least in part, upon the temperature at which the metal nanoparticlesare formed. Moreover, the length of time that the reduction is allowedto proceed can also influence the size of the metal nanoparticlesformed. At lower temperatures, the reaction to form metal nanoparticlescan be completed more slowly (several hours), whereas at highertemperatures a very rapid reaction can occur (a few minutes). In someembodiments, the reaction can take place over a period of about 1 toabout 3 hours, and in other embodiments, the reaction can take placeover a period as short as 1 minute. Relatively short reaction times canbe desirable for industrial scale processes.

While reacting the reducing agent with the second metal salt, thereaction mixture can be monitored for signs of metal nanoparticleformation. These signs can include, for example, a change in colorand/or gas evolution. Metal nanoparticle formation can also be monitoredcolorimetrically, photometrically, or potentiometrically, if desired.

Once a desired amount of reduction to form metal nanoparticles has takenplace, the reaction can be stopped in various ways. In some embodiments,the reducing agent can be quenched with a terminating agent that isreactive with the reducing agent but not substantially reactive with themetal nanoparticles. Illustrative examples of terminating agents caninclude, for example, aldehydes, ketones, nitriles, organic acids,water, combinations thereof, and the like. More specific examples caninclude, for example, acetaldehyde, acetonitrile, formic acid, aceticacid, malic acid, and oxalic acid. One of ordinary skill in the art willrecognize that by quenching the reducing agent, one can arrest theformation of metal nanoparticles. In some embodiments, the amount of theterminating agent can be chosen such that it is stoichiometricallyequivalent with the amount of excess reducing agent.

In other embodiments, the formation of metal nanoparticles can bearrested by means other than or in combination with quenching of thereducing agent. For example, in some embodiments, the formation of metalnanoparticles can be arrested by cooling the reaction mixture to about−10° C. or below (e.g., in a liquid nitrogen or dry ice bath),centrifuging the reaction mixture, or combinations thereof. One ofordinary skill in the art will recognize that in the former case,lowering of the reaction temperature to this degree will effectivelystop the reduction process. In the latter case, centrifuging the metalnanoparticles can remove them from the reaction mixture in which thereducing agent is present, thereby separating the metal nanoparticlesfrom the excess reducing agent. In some embodiments, centrifuging themetal nanoparticles can take place at a reduced temperature (e.g., belowroom temperature) so as to minimize the risk of nanoparticle fusionoccurring during the centrifugation process. For example, in someembodiments, centrifugation can take place at a temperature rangingbetween about −10° C. and about 15° C.

In some embodiments, the metal nanoparticles formed by the methodsdescribed herein can be used in situ without further isolation. In otherembodiments, a work up of the reaction mixture can be performed topurify and isolate the metal nanoparticles. Isolation and purificationof the metal nanoparticles can include a series of rinses, sonication,and centrifugation steps. After isolation of the metal nanoparticlesfrom the reaction mixture, various purification processes can optionallybe conducted. For example, after isolation of the metal nanoparticles bycentrifugation, the mother liquor can be decanted, and the metalnanoparticles can be washed with glyme, THF, or a like solvent to removeexcess reducing agent and potential organic side reaction products.Next, water washes can then be conducted until an AgNO₃ test of the washwater is negative for the presence of chloride, which typically takesabout 1 to about 5 water washes. In some embodiments, aqueous ammoniawashes of the metal nanoparticles can be conducted to remove anyunreacted metal salt from the reaction mixture. In such embodiments, aratio of about 1:4 to about 1:10 ammonia:water can most typically beused.

In some embodiments, after their preparation and isolation, the metalnanoparticles can be stored under conditions that facilitate their longterm stability. In some embodiments, the metal nanoparticles can bestored under water or another solvent that contains an antioxidant suchas, for example, citric acid, ascorbic acid, butylated hydroxyanisole(BHA), butylated hydroxytoluene (BHT), or propyl gallate (PG). In someembodiments, the metal nanoparticles can be stored under long chainhydrocarbons (e.g. mineral oil), high boiling petroleum ether and thelike. In some embodiments, the metal nanoparticles can be stored bysealing a powder of metal nanoparticles with a wax layer and storing ina closed container. In some embodiments, the container housing the metalnanoparticles can be flushed with an inert gas such as nitrogen or argonbefore being sealed so as to further minimize the risk of oxidationduring storage. In some embodiments, the storage container can be sealedwith, for example, a wax layer, shrink wrap, tape, film, and the like.

In some embodiments, the metal nanoparticles can be dispersed in amaterial that prevents the metal nanoparticles from coming in contactwith one another and optionally protects the metal nanoparticles fromatmospheric oxygen during storage. Illustrative materials meeting thesecriteria can include, for example, waxes, long chain amines havinggreater than about 10 carbon atoms, paraffins, and aromatic compoundssuch as, for example, phenanthrene, pyrene, or anthracene.

In some embodiments, the metal nanoparticles can be mixed with anadditive such that they have a desired consistency. For example, themetal nanoparticles can be formulated to produce the consistency of agel, paste, paint or like material. In such embodiments, the metalnanoparticles can be formulated and readily applied to a surface. Forexample, in some embodiments, the metal nanoparticles can be used toform a connection on a surface (e.g., to join a first member to a secondmember). In some embodiments, the metal nanoparticles can be mixed withan additive such as, for example, dicyclohexylamine, paraffin wax,glycerin, or flux materials (e.g., formic acid, acetic acid,hexadecanoic acid, or triethanolamine) to attain a desired consistency.In some embodiments, small amounts of solvents such as, for example,ethanol, isopropanol, butanol, t-butanol, cyclohexanol, acetone, tolueneand the like can be used to obtain a consistency suitable for readyapplication to a surface. In some embodiments, a solution or suspensionof the metal nanoparticles can be partially evaporated to remove atleast some of the solvent therefrom to attain a desired consistency.When formulated as a gel, paste, paint or the like, about 75% to about98% metal nanoparticles by weight are usually present, with the balancebeing solvent and/or additives used to attain the desired consistency.However, for these applications and others, nanoparticle concentrationseven as low as about 50% by weight can be used. Particularly suitableadditives and combinations thereof for dispensing metal nanoparticlescan include those described in commonly owned U.S. patent applicationSer. No. 13/764,669, filed on Feb. 11, 2013 and incorporated herein byreference in its entirety.

In alternative embodiments, the insoluble compound constituting thenanoparticle seeds can be converted directly to metal nanoparticles.Specifically, in some embodiments, methods for forming metalnanoparticles can include reacting the nanoparticle seeds with areducing agent in the presence of at least one surfactant in order toform the metal nanoparticles. In some embodiments, a copper (I)nanoparticle seed can be directly reduced to form copper nanoparticles.In some embodiments, methods for forming copper nanoparticles caninclude providing a copper (II) salt in a solvent; reducing the copper(II) salt to form an insoluble copper (I) salt having a particle size ofabout 10 nm or less; after forming the insoluble copper (I) salt, addingat least one surfactant and a reducing agent thereto; and reacting thereducing agent with the insoluble copper (I) salt in the presence of theat least one surfactant to form a plurality of copper nanoparticles.Other metal nanoparticles can be synthesized similarly. The conversionof nanoparticle seeds to metal nanoparticles in this manner canadvantageously take place without an increase in size relative to thatof the nanoparticle seeds. The uniqueness of this approach is thatcopper is not being reduced from a solution, as in other processesdescribed herein, but instead from a suspension or sol of nanoparticleseeds.

In some embodiments, compositions containing metal nanoparticles aredescribed herein. In various embodiments, the metal nanoparticles canhave a metal shell formed around a nucleus. The nucleus can be derivedfrom an insoluble compound that is produced as described above. In someembodiments, the nucleus can contain a metal that is different than themetal shell surrounding it. In other embodiments, the nucleus cancontain the same metal as the metal shell. Illustrative metals that canbe present in the metal shell include transition metals (Sc, Ti, V, Cr,Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W,Re, Os, Ir, Pt, Au), main group metals (e.g., Al, Ga, In, Tl, Sn, Pb,Sb, Bi), and lanthanide metals (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho,Er, Tm, Yb, Lu). The metal nanoparticles can be formed by the processesdescribed hereinabove.

In more particular embodiments, the metal nanoparticles can contain anucleus derived from a copper (I) salt. That is, the nanoparticle seedscan be formed from a copper (I) salt such as, for example, copper (I)chloride, copper (I) bromide, or copper (I) iodide, and the copper (I)nanoparticle seeds can be subsequently converted during the formation ofmetal nanoparticles. In such cases, suitable copper (II) salts used forforming the copper (I) nanoparticle seeds can include copper (II)chloride, copper (II) bromide, or copper (II) iodide. Suitable copper(II) salts are not limited to halides and can also include other copper(II) salts such as, for example, copper (II) sulfate, copper (II)acetate, copper (II) nitrate, and the like. In other embodiments, themetal nanoparticles can contain a nucleus formed from an iron compound,a silver compound or a gold compound. In still other embodiments, themetal nanoparticles can contain a nucleus formed from Cu₂O. In the caseof silver and gold, the nucleus can already be in a metallic state priorto formation of a metal shell around it. When not already in a metallicstate, the nanoparticle seeds can be reduced to a metallic state whileforming the metal shell around the nucleus. As described above, othersubstances can also form the nucleus of the metal nanoparticlesdescribed herein.

In some embodiments, the metal nanoparticles can be about 30 nm or lessin size, or about 20 nm or less in size, or about 10 nm or less in size,or about 5 nm or less in size. In some embodiments, the metalnanoparticles can range between about 0.1 nm and about 30 nm in size, orbetween about 0.5 nm and about 5 nm in size, or between about 1 nm andabout 10 nm in size, or between about 5 nm and about 20 nm in size. Insome embodiments, the metal nanoparticles can range between about 0.5 nmand about 6 nm in size, or between about 1 nm and about 6 nm in size, orbetween about 3 nm and about 6 nm in size, or between about 3 nm andabout 5 nm in size, or between about 2 nm and about 6 nm in size.

In some embodiments, mixtures of metal nanoparticles having differentsize distributions can be used in the compositions described herein.Specifically, in some embodiments, mixtures of metal nanoparticles canhave a first plurality of metal nanoparticles having a first size rangeand a first average size and a second plurality of metal nanoparticleshaving a second size range and a second average size. In someembodiments, the mixture of metal nanoparticles can have a bimodal sizedistribution. For example, in some embodiments, metal nanoparticleshaving sizes ranging between about 0.5 nm and about 5 nm can be mixedwith metal nanoparticles having sizes ranging between about 10 nm andabout 25 nm. Given the benefit of the present disclosure, one ofordinary skill in the art will be able to determine whether a singlepopulation of metal nanoparticles or mixture of metal nanoparticleshaving different sizes is best suited for a given application. Forexample, a mixture of metal nanoparticles having different fusiontemperatures can be used when it is desired to liquefy only a portion ofthe metal nanoparticles.

Metal nanoparticles produced by the techniques described herein can beused in a number of applications. In some embodiments, the metalnanoparticles can be used in soldering applications. More particularly,the metal nanoparticles can be applied to a substrate and then at leastpartially fused together to join two or more surfaces together. In someor other embodiments, the metal nanoparticles can be used inapplications such as printed circuit boards (PCBs), conformal coatings,thin film solar cells, batteries, drug delivery and drug therapysystems, thermoelectric materials, gas sensors, conductive inks, and thelike.

EXAMPLES

Tin nanoparticle syntheses (Examples 1 and 2) were carried out underinert gas atmosphere, and all solvents and surfactants were degassedwith dry argon or nitrogen gas for 5 minutes before use.

Example 1: 40 ml of water was placed into a 100 mL 3 neck round bottomflask, which was then evacuated and backfilled with argon three times.Thereafter, 0.5 g of copper (II) chloride dihydrate was added, and themixture was stirred for 30 minutes at room temperature until completelydissolved. A 50 mL aqueous solution containing 2.5 g of tin (II)chloride dihydrate was then added rapidly to the copper (II) chloridesolution. The reaction mixture turned opaque immediately due to theformation of copper (I) chloride seeds (see FIG. 3A). To the copper (I)chloride seeds, the following surfactants were added under positiveargon pressure: 2 mL of n-butylamine or t-butylamine and 3 mL ofpyridine.

A 50 mL round bottom flask was charged with 12 mL of basic (0.5 mLn-butylamine) 2.0 M sodium borohydride solution. While keeping bothflasks at room temperature, the sodium borohydride solution wastransferred to the flask containing the copper (I) chloride seeds overno more than 30 seconds using a cannula or syringe. The reaction mixtureimmediately turned black and evolved gas. Once the reaction wascomplete, as evidenced by the cessation of gas evolution, the reactionmixture was cooled to 0° C. in an ice bath for 10 minutes. The reactionmixture was centrifuged at 2200 RPM for 10 minutes, resulting in a blackprecipitate and a clear supernatant. The black precipitate was washedwith a dicyclohexylamine (4 mL)/water mixture (40 mL). The mixture wasagain centrifuged at 2200 RPM for 10 minutes, resulting in a black togrey precipitate and clear supernatant. The solid was then stored in aclosed container under argon.

Example 2: 75 mL of triglyme was placed in a 100 mL 3-neck round bottomflask, which was then evacuated and backfilled with argon three times.Thereafter, 0.5 g of anhydrous copper (II) chloride was added, followedby the following surfactants under positive argon pressure: 2 mL ofn-butylamine and 3 mL of pyridine. The reaction mixture was stirred for1 hour at 45° C. until the solids were completely dissolved. Thereafter,2.5 g of SnCl₄ was added, followed rapidly by 20 mL of a triglymesolution containing 0.5 g of dissolved glucose. The reaction mixtureturned opaque due to the formation of the nanoparticle seeds.

A 50 mL round bottom flask was charged with 25 mL of dry 2.0 M sodiumborohydride solution in triglyme. While keeping both flasks at 45° C.,the sodium borohydride solution was transferred to the flask containingthe seeded tin (IV) chloride solution over no more than 30 seconds usinga cannula. The reaction mixture immediately turned black and evolvedgas. Once the reaction was complete, as evidenced by the cessation ofgas evolution, the reaction mixture was cooled to 0° C. in an ice bathfor 10 minutes. The reaction mixture was centrifuged at 2200 RPM for 10minutes, resulting in a black precipitate and a clear supernatant. Theblack precipitate was washed with a dicyclohexylamine (4 mL)/watermixture (40 mL). The mixture was again centrifuged at 2200 RPM for 10minutes, resulting in a black to grey precipitate and clear supernatant.The solid was then stored in a closed container under argon.

Example 3: 230 mL of triglyme was placed in a 500 mL 3-neck round bottomflask, which was then evacuated and backfilled with argon three times.Thereafter, 4 g of anhydrous copper (II) chloride was added, followed bythe following surfactants under positive argon pressure: 12 mL oft-octylamine and 20 mL diisobutylamine. The reaction mixture was thenstirred at 42° C. until the solids were completely dissolved. 15 mLN,N′-di-t-butylethylenediamine was then added, and the reaction mixtureturned opaque with the formation of nanoparticle seeds in the form of aninsoluble ligand complex.

Copper nanoparticles were then formed directly from the nanoparticleseeds. A 100 mL round bottom flask was charged with 50 mL of dry 2.8 Msodium borohydride solution in triglyme. The sodium borohydride solutionwas transferred to the flask containing the nanoparticle seeds over nomore than 30 seconds using a cannula. The reaction mixture turned blackand evolved gas over the next 5-10 minutes. Once the reaction wascomplete, as evidenced by the cessation of gas evolution, the reactionmixture was cooled to 0° C. in an ice bath for 10 minutes. The reactionmixture was centrifuged at 2200 RPM for 10 minutes, resulting in a blackprecipitate and a clear supernatant. The black precipitate was thenwashed with a hexylamine/water mixture (4 mL/40 mL). The mixture wasagain centrifuged at 2200 RPM for 10 minutes, resulting in a clearsupernatant and a dark brown precipitate with a copper sheen. The solidwas collected and stored in a closed container under argon.

Example 4: 230 mL of triglyme was placed in a 500 mL 3-neck round bottomflask, which was then evacuated and backfilled with argon three times.Thereafter, 3 g of anhydrous silver (I) nitrate and 12 mLN,N′-di-t-butylethylenediamine were added under positive argon pressure.The reaction mixture was then stirred until the solids were completelydissolved. Thereafter, 10 mL of benzoic acid was added, upon which thereaction mixture turned opaque with the formation of nanoparticle seeds.

Silver nanoparticles were then formed directly from the nanoparticleseeds. A 100 mL round bottom flask was charged with 20 mL of dry 2.8 Msodium borohydride solution in triglyme. The sodium borohydride solutionwas transferred to the flask containing the nanoparticle seeds over nomore than 30 seconds using a cannula. The reaction mixture turned blackand evolved gas over the next 2-3 minutes. Once the reaction wascomplete, as evidenced by the cessation of gas evolution, the reactionmixture was cooled to 5° C. in an ice bath for 10 minutes. The reactionmixture was then centrifuged at 2200 RPM for 7 minutes, resulting in ablack precipitate and a clear supernatant. The black precipitate waswashed with a hexylamine/water mixture (4 mL/40 mL). The mixture wasagain centrifuged at 2200 RPM for 7 minutes, resulting in a blackprecipitate and clear supernatant. The solid was then collected andstored in a closed container under argon.

Example 5: 0.07 g dry bismuth (III) chloride and 0.05 g dry sodiumtelluride were placed in separate 100 mL 3-neck round bottom flasks,each containing 50 mL of methanol, and dissolved in the solvent.Thereafter, the solutions were cooled to −20° C. using a standardchiller and then combined by cannulating them simultaneously into athird 500 mL 3-neck round bottom flask containing 25 mL of ethanol, alsoat −20° C. Upon warming to 0° C., a dark, opaque mixture of nanoparticleseeds formed.

To the nanoparticle seeds, the following two solutions were addedsimultaneously at 0° C.: one containing 0.7 g lanthanum chloride in 50mL ethanol and the other containing 0.5 g of sodium telluride in 50 mLethanol. After slowly warming to room temperature, the reaction mixtureturned black. Thereafter, 20 mL of diethyleneglycol diethylether wasadded. The reaction mixture was then centrifuged at 2200 RPM for 7minutes, resulting in a black precipitate and a clear supernatant. Theblack precipitate was washed with a diglyme/water mixture (4 mL/40 mL).The mixture was again centrifuged at 2200 RPM for 7 minutes, resultingin a black to grey precipitate and clear supernatant. The solid was thencollected and stored in a closed container under argon.

Although the invention has been described with reference to thedisclosed embodiments, those skilled in the art will readily appreciatethat these only illustrative of the invention. It should be understoodthat various modifications can be made without departing from the spiritof the invention. The invention can be modified to incorporate anynumber of variations, alterations, substitutions or equivalentarrangements not heretofore described, but which are commensurate withthe spirit and scope of the invention. Additionally, while variousembodiments of the invention have been described, it is to be understoodthat aspects of the invention may include only some of the describedembodiments. Accordingly, the invention is not to be seen as limited bythe foregoing description.

1. A method comprising: forming a plurality of nucleation seeds as aninsoluble compound in a reactor in the presence of a first metal salt,the first metal salt being reducible to form a zero-valent metal; afterformation of the plurality of nucleation seeds is complete, adding areducing agent to the reactor and reducing the first metal salt with thereducing agent to form the zero-valent metal in the presence of at leastone surfactant; and growing a plurality of metal nanoparticles in whicheach metal nanoparticle comprises a shell comprising the zero-valentmetal formed around a nucleus derived from a nucleation seed.
 2. Themethod of claim 1, wherein the metal shell has the at least onesurfactant bonded thereto.
 3. The method of claim 1, wherein a secondmetal salt is reduced in the reactor to form the plurality of nucleationseeds.
 4. The method of claim 1, wherein a portion of the first metalsalt is reduced in the reactor to form the plurality of nucleationseeds.
 5. The method of claim 4, wherein the portion of the first metalsalt is reduced with a second metal salt or a reducing sugar to form theplurality of nucleation seeds.
 6. The method of claim 4, wherein thefirst metal salt is a copper (II) salt.
 7. The method of claim 6,wherein the plurality of nucleation seeds comprise copper (I) chloride.8. The method of claim 1, wherein the first metal salt is a basic copper(II) salt stabilized with ammonia, citrate, or tartrate, and theplurality of nucleation seeds are formed by reacting the basic copper(II) salt with a reducing sugar; wherein the plurality of nucleationseeds comprise Cu₂O.
 9. The method of claim 1, wherein the plurality ofnucleation seeds comprise a metal carboxylate.
 10. The method of claim1, wherein the plurality of nucleation seeds comprise a transition metalsulfide.
 11. The method of claim 1, wherein the plurality of nucleationseeds comprise an insoluble metal-ligand complex.
 12. The method ofclaim 1, wherein the plurality of nucleation seeds comprise a sol. 13.The method of claim 1, wherein the plurality of nucleation seedscomprise colloidal sulfur.
 14. The method of claim 1, wherein theplurality of nucleation seeds comprise a Group II carbonate or a GroupII sulfate.
 15. The method of claim 1, wherein the plurality ofnucleation seeds comprise a telluride or selenide.
 16. The method ofclaim 1, wherein the metal nanoparticles are about 30 nm or less insize.
 17. The method of claim 1, wherein the plurality of nucleationseeds are about 3 nm or less in size.